A parametric, or nonlinear, sonar utilizes the nonlinear compressibility of water to generate a narrow sonar beam having unusually low sidelobes. In the prior art systems, two intense high-frequency beams are transmitted simultaneously into the water where they interact nonlinearly to produce sum-frequency and difference-frequency waves. The "sum" wave dominates the pressure field in the water over short ranges; the "difference-frequency" wave survives over long ranges because the two primary sine waves, and especially the "sum" wave, attenuate rapidly with distance. "Difference-frequency" wave effects resulting from the system transducer array or electronics, however, are undesirable where a narrow nonlinear beam is sought because such effects lead to broad beam pattern radiation.
This problem has been addressed by prior art systems which have carefully summed and amplified the constituent signals before entering the "sum" wave component signals into a transducer. However, the cost of including a linear amplifier which screens out the "difference-frequency wave, the space it requires, and the possibility of the difference frequency being generated, nonetheless, thereby masking the narrow beam, make the prior art parametric sonar systems less than optimal.
U.S. Pat. Nos. 3,924,259 and 3,824,531 represent prior art apparatuses which combine two signals in order to produce sonar energy at a ". . .resultant frequency equal to an arithmetic combination of the radiated frequencies. . ." of the two signals. Once formed, the composite signal enters various elements, including a clipping circuit, which convert the combined analog signal into digital form. The digitizing makes the composite signal compatible with digital multiplexing switches and computer commands found in subsequent beamsteering stages. The commands from the computer are designed to delay the respective outputs from each of a plurality of arrays in order to form a beam generated at the resultant frequency. The large amount of equipment required to generate, amplify, and compute the required delay represents a significant disadvantage in such prior art systems.
In U.S. Pat. Nos. 3,697,994 and 3,683,386 beamsteering systems employing a digital computer, together with interconnected adders, are taught. The apparatuses disclosed therein control the radiations of energy from array elements in a transducer by use of phase information only. Using phase only (and not time), sufficient information to steer a sonar beam where ensonified regions are spaced at distances comparable to the wavelength of the sonar signal is not provided. The digital phase shifting techniques of these references have been less than optimal in certain applications.